Amputation of the arm or leg causes significant disability, the most effective treatment for which is replacement of the missing limb with a prosthetic device. However, amputations that result in short residual limbs pose significant problems for fitting and suspension of a prosthetic device, and control of the prosthesis is compromised because of the short length of the lever arm provided by the residual limb. This results in poor energy transfer between the limb and an attached prosthesis, as short lever arms generate less torque for a given force. This functional deficit is compounded when the lever arm is encased in very compliant tissue, such as a residual femur that is surrounded by the soft tissues of the thigh, which further impairs prosthesis control. Individuals with short residual limbs following leg amputation display greater gait asymmetries and gait changes than those with longer residual limbs (Bell et al. 2013). In addition, short residual legs reduce stability and balance when sitting and when performing transfers, such as getting on and off the toilet or into a car. Postural changes and compensatory mechanisms associated with use of short residual limbs can cause discomfort and injury to the spine or other body structures.
Lengthening of a limb can be accomplished through Distraction Osteogenesis (DO), a process that starts by performing an osteotomy; a surgical procedure wherein a bone is cut into two segments, a proximal segment (nearer to the body) and a distal segment (further from the body). The two segments are then gradually separated by an applied traction force at an expansion rate such as 1 mm per day (Ilizarov 1990). After the bone has been cut, new bone starts to form in the resulting gap as a result of natural bone fracture-healing mechanisms. The period when new bone is forming is known as the distraction phase. When the desired length is reached, the distraction force is discontinued and the new bone ossifies and remodels into mature bone during the consolidation phase (Samchukov 2008).
Current limb lengthening procedures that use DO rely on external devices (Ilizarov 1989) or fixators, which must be worn a minimum of 1 day for distraction and 2 days for consolidation for every millimeter of length gained, for a total of at least 3 days per millimeter of additional length. For example, to gain 75 mm of length, the patient would have to wear such a device for at least 225 days, or 7.5 months. Frequently, complications extend this timeline, such that it can sometimes be almost twice as long (Sakurakichi et al. 2002). Additionally, some external components of these devices must pass through the skin and attach to the bone at a minimum of 4 places, creating 4-8 percutaneous wounds that pose a considerable risk of infection. Thus an additional complication associated with use of these devices is infection and scarring at sites of skin penetration. Such complications are reported in up to 59% of patients (Paley 1990). Some studies show skin infection rates as high as 78-100% (Antoci et al. 2008). Infection rates decrease with fewer points of skin penetration (Brewster et al. 2010).
Because of their design, traditional external fixator DO devices are large and cumbersome. They interfere with the ability to wear clothing and to move around in bed, which affects sleep. The Ilizarov technique requires a cage that encircles the limb so that the patient must keep their limb in an abducted position, which is uncomfortable, reduces mobility, interferes with the use of crutches or wheelchairs, and impairs general function. External fixators, which are placed on the lateral side of the leg, also interfere with the use of crutches, use of a wheelchair, and general function.
The prior art devices preclude an active lifestyle for several months, which may contribute to physical decline, depression, and other psychological consequences. Despite the advantages provided by a longer residual limb, the inordinate burden imposed on the patient by these devices and the relatively high rates of complication often limit the use of DO to lengthen residual limbs in amputees. Utilization of the prior art is often too difficult, physically and emotionally, for a patient with an amputation.
Lengthening-over-a-nail (LON) is a technique for patients with intact limbs that decreases the time a patient needs to wear the external fixator device. During the distraction phase, the LON approach utilizes both an external fixator and an internal nail inserted into the medulla of the bone, known as an intramedullary nail or IM nail. The external device is removed during the consolidation phase; however, the IM nail remains in the bone to provide support. By decreasing the required wear time of the external fixator device, LON techniques have led to decreased rates of superficial infection from 36.2% with traditional external fixator devices to 1.4% with LON (Brewster et al. 2010). Thus, limiting the time that external, percutaneous components are required can decrease overall complications in addition to improving patient comfort and quality of life (Kocaoglu et al. 2004; Mahboubian et al. 2012).
Several intramedullary devices for DO exist for patients (usually children) with intact limbs. The Intramedullary Skeletal Kinetic Distractor (ISKD) (Cole et al. 2001) (Orthofix Inc., Texas) is the only FDA-approved intramedullary DO device available in the United States at this time. These devices allow bone lengthening in patients with intact limbs, without the need for any percutaneous devices. However, the ISKD has been shown to produce variable rates of distraction, which increases the risk of non-union, pseudoarthrosis, or early consolidation (Mahboubian et al. 2012). Reviews of this and other devices have proven that poorly controlled distraction (e.g., lengthening>1.5 mm/day, so-called runaway) is an important risk factor for poor bone formation (Kenawey et al. 2011).